Various processes for the preparation and/or purification of Malathion are disclosed in the literature. Malathion {CAS Number: 121-75-5} is an organophosphate insecticide that inhibits cholinesterase activity in vivo. The U.S. Food and Drug Administration (FDA) supports the pharmaceutical use of malathion for the treatment for head lice in children. Due to its low toxicity to humans. Malathion may be prepared by reacting O,O-dimethyldithiophosphoric acid (OODMDTPA) with diethyl maleate (U.S. Pat. Nos. 2,578,652, 2,879,284, 3,403,201, 3,463,841, 3,470,272, 4,367180, 2007/0010496 and 4,681,964).

But, still numerous impurities are found in Malathion preparation. Some of these impurities are formed during storage and some are generated during the manufacturing process. Many of these Malathion impurities have been found to be toxic. O,O,S-trimethyl phosphorodithioate (MeOOSPS) and O,S,S-trimethyl phosphorothioate (MeOSSPO) can cause liver damage (Keadtisuke et al, Toxicology letters 52:35-46 (1990) or immune suppression (Rodgers et al, immunopharmacology 17: 131-140 (1989). The toxicity of Iso Malathion is due to its ability to inhibit acetylcholinesterase, in fact Iso Malathion is ˜1,000 times as active against acetylcholinesterase as compared with Malathion (Tetrahedron letters 33(11). 1415-18 (1992). Malathion physical properties make it difficult to remove impurities by conventional means for example, because Malathion is liquid at ambient temperature (melting point: 2.9° C.), and crystallization is difficult.
Therefore, we have developed an improved process for the preparation of Malathion for pharmaceutical use. Malathion produced by this method has significantly lower levels of toxic impurities and storage stable, when compared to the any other commercial method available in literature for the pharmaceutical use.